Field
The present invention relates to a method of forming a structural connection between a spar and an aerodynamic fairing and in particular to a spar and an aerodynamic fairing for a wind turbine blade.
Description of the Related Art
Modern wind turbine blades are typically made by separately manufacturing a structural beam or spar which extends along the length of the blade and two half shells, or aerodynamic fairings, which are attached to the spar to define the aerodynamic profile of the blade.
A typical method of attaching the fairings to the spar is shown in FIGS. 1A-1C. In this method, an adhesive 2 is applied to the upper and lower surfaces of the spar 4 and the fairings 6 are placed over the adhesive 2 to enclose the spar 4, as shown in FIG. 1B. Typically, a cavity with a thickness of between 5 mm and 50 mm will exist between the upper and lower surfaces of the spar 4 and fairings 6, and the adhesive 2 fills this cavity. The fairings 6 are then clamped against the spar 4 to squeeze the adhesive 2 and bond the fairings 6 to the spar 4. As the forces required to squeeze the adhesive 2 are high, the fairings 6 are typically kept in the tools in which they were formed for the step of bonding each fairing 6 to the spar 4.
However, as the fairings 6 are pressed against the spar 4, the adhesive applies pressure to the inner surface of the fairings 6. This can lead to distortion of the fairings 6 and the tools (not shown) in which they are held, resulting in a distorted aerodynamic surface 8, as shown in FIG. 1C. Thus, the tools must be of sufficient structural stiffness to prevent such distortion.
Although acceptable for smaller blades, the above technique can be extremely expensive when used to bond larger blades, such as those which are 45 meters or more in length. This is due to the cost of building a tool large enough to accommodate the fairings, stiff enough not to distort, and which can be lifted and closed accurately.
US 2009/0226702 is directed to an adhesive joint for use in joining various wind turbine components. This document recognises a problem with excessive adhesive used in these joints. In particular, it is not possible to remove this excess adhesive in a closed structure. Such excess adhesive may break off in use and cause problems such as clogging drainage holes and causing damaging impact forces. In order to overcome this problem, this document proposes providing a porous layer which extends beyond the adhesive joint. Once the space in the region of the joint around the porous member is fully filled with adhesive, excess adhesive will be squeezed into the parts of the porous layer outside of the joint. It is then retained during use within this porous layer and the problems of the loose lumps of adhesive are solved. The document does not address the assured distortion of the fairings and tools which is addressed by the present invention.